a) Field of the Invention
This invention relates to a control system for an electronically-controlled automatic transmission, which permits determination of a full stop of a vehicle.
b) Description of the Related Art
The speed of a vehicle has heretofore been computed, on the basis of output pulses from a vehicle speed sensor, by a central processing unit (CPU) in an electronic control system. For example, using an electromagnetic pickup coil, a pulse is generated by passage of a metal-made rotor through a magnetic field produced by an electromagnetic coil. The pulse is detected as a sinusoidal wave by the vehicle sensor as shown in FIG. 8, is converted by a vehicle speed input processor into a square wave as depicted in FIG. 9, a leading edge interval T.sub.1 of the square wave is detected by the central processing unit (CPU) in the electronic control system, and vehicle speed is then computed based on the leading edge interval T.sub.1.
For detection of a full stop of the vehicle, on the other hand, the vehicle has been regarded to be at a full stop when the vehicle speed pulse interval T is equal to or longer than a predetermined constant time T'.sub.stop, in other words, when the condition of T'.sub.stop .ltoreq.T is met.
The method for the above determination of the full stop of the vehicle is illustrated in FIG. 10 which is a flowchart for the conventional determination of the full stop of the vehicle.
(1) First, a vehicle speed pulse interval T.sub.1 is detected (step S1).
(2) Next, vehicle speed is computed based on the vehicle speed pulse interval T.sub.1 (step S2).
(3) Then, it is determined whether or not a detected vehicle speed pulse interval T is equal to or longer than the predetermined constant time T'.sub.stop, in other words, satisfies T'.sub.stop .ltoreq.T ( step S3).
(4) Where T'.sub.stop .ltoreq.T is determined above, the vehicle is judged to have stopped so that vehicle-stop-time processing is performed (step S4).
Incidentally, with the shift lever in a drive position D, when a vehicle having an automatic shifting system is stopped (with the engine idling), due to a dragging torque in the torque converter, the so-called creep phenomenon causes the vehicle to move forward.
To prevent the creep phenomenon, in other words, to perform non-creep (anti-creep) control, it is necessary to precisely detect a full stop of the vehicle.
Examples of such non-creep control systems for vehicles include the control system disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 60-60348.
Determination of a full stop of a vehicle according to the above-described conventional vehicle-stop time processing, however, is delayed by a time T'.sub.stop as illustrated in FIG. 12 although, as shown in FIG. 11, the vehicle has already been at a full stop at point "a" insofar as the actual vehicle speed is relied upon.
Further, the use of the so-called electromagnetic pickup coil system as a vehicle speed sensor, wherein a pulse is generated as a result of passage of the metal rotor through a magnetic field, involves the inherent problem that when the vehicle speed is very low, the detection of the vehicle speed by the vehicle speed sensor is practically impossible because no pulse is generated from the electromagnetic coil unless the speed of the rotor passing through the magnetic field is equal to or higher than a predetermined value.
Even if the results of plural pulse detecting operations are averaged as in the prior art, high-accuracy detection of a vehicle speed is impossible in a vehicle speed range close to 0.
The vehicle speed is 0 could be taken as a point at which a predetermined time has elapsed since stop of pulse generation as described above. In this case, however, the predetermined time varies depending on the manner of the stop of the vehicle, thereby making it impossible to determine a full stop of the vehicle as a point in time concurrent with the actual full stop of the vehicle.
More specifically, cessation of pulse generation precedes by a short time the actual stop of the vehicle when the vehicle is abruptly stopped. However, the discrepancy becomes longer when the vehicle is gradually stopped. If the above-described predetermined time was set constant, the determination of a stop would be delayed for an abrupt stop but, in the case of a gradual stop, the vehicle would be determined to be at a stop prior to the actual stop.
Such determination of a stop of a vehicle is important in non-creep (anti-creep) control, thus creating demand for precise detection of a full stop of the vehicle and hence accurate control of the vehicle.